In the field of electro-optics, a non-reflective coating is frequently needed. Such coating should, preferably, be completely non-reflecting to an arriving beam.
To reduce mode coupling and resonator losses in ring laser gyros having intracavity elements such as Faraday Rotators, it is necessary to reduce reflections from such elements, and preferably to reduce reflections to zero. In designing an anti-reflective coating, one might specify both substrate and dielectric film materials having certain required indices of refraction, and such materials might be unavailable. In the real world there are very few low-loss coating materials and even fewer materials that are also sufficiently stable in the environment of a ring laser gyro.
In a multisensor, for example, the Faraday Cell may be fabricated of SF-57, a Schott glass with a high Verdet constant. A non-reflective or anti-reflective coating is deposited on the surfaces of the Faraday Cell.
A typical anti-reflective coating on the Faraday Cell, to prevent reflections of laser beams back into the ring laser, may be a multifilm dielectric of silica, SiO.sub.2, and titania, TiO.sub.2. The reflectivity of the multifilm coating depends upon the thicknesses and the relative thicknesses of the two films. For a general discussion of the reflectivity of multifilms, see, "Principles of Optics," fifth edition, by Max Born and Emil Wolf, Pergamon Press, 1975, pages 59 et. seq.
Retroscatter from surfaces in a ring laser gyro induces frequency and intensity variations of the individual counter propagating beams. Both the frequency variation, which is sinusoidal with a frequency equal to the beat frequency between the beams, and the intensity variation produce a locking of the two beam frequencies, and the ring laser gyro can no longer measure angular rotation.